1. Field of the Invention
The present invention relates to a magnetic head drum or cylinder for use in a helical-scan magnetic recording and reproducing apparatus such as a VTR, an R-DAT, or the like.
2. Description of the Prior Art
In VTRs, R-DATs, or similar helical-scan magnetic recording and reproducing apparatus, desired information is recorded on and reproduced from magnetic tapes by rotary magnetic heads. The rotary magnetic heads are supported on a rotatable head drum or cylinder. Heretofore, the head drum is made of an aluminum alloy. However, since the magnetic tape is held in sliding contact with the outer circumferential surface of the head drum assembly, the head drum tends to be worn rapidly, particularly at a lead edge for guiding an edge of the magnetic tape. Such excessive wear of the head drum causes a compatibility problem.
In view of the above drawback, it was once customary to employ head drums of a commercially available aluminum alloy such as "AC8B" which is an Al-Si-Cu-Mg alloy, or "AHS" which is an eutectic malleable material containing 10 to 20% by weight of Si. However, since no sufficient durability is achieved by the above aluminum alloys (AC8B, AHS), present head drums are made of the powder of an Al-Si alloy containing 20 to 30% by weight of Si.
AC8B is defined in the Japanese Industrial Standards as containing the following percent by weight ranges of constituents. Cu 2.0 to 4.0, Si 8.5 to 10.5, Mg 0.5 to 1.5, Zn 0.5, Fe 1.0, Mn 0.5, Ni 0.1 to 1.0, Ti 0.2, Pb 0.1, Sn 0.1, Cr 0.1. AHS is defined in the Japanese Industrial Standards as containing in percent by weight Si 11.0 to 13.5, Fe 1.0, Cu 0.5 to 1.3, Mg 0.8 to 1.3, Cr 0.1, Ni 0.5 to 1.3, Zn 0.25.
Japanese Laid-Open Patent Publication No. 56-156956 discloses a head drum which has a surface for sliding contact with a magnetic tape, the surface being provided by a surface layer that is made of a wear-resistant material different from the material of the head drum proper. The surface layer is joined to the head drum proper by a pressure fit, a shrink fit, adhesive bonding, insert molding, or the like.
The head drums which are made of the powder of an Al-Si alloy containing 20 to 30% by weight of Si are disadvantageous in that the cost of the material is 4 to 5 times higher than ordinary aluminum alloys. In addition, since the entire head drums are made of a wear-resistant material, they cannot easily be machined by cutters, drill bits, and tapping screws, or require many machining steps until they are finished.
With the head drum having a surface layer, it is necessary to fabricate the surface layer separately, and also to join the surface layer to the head drum proper by a pressure fit, a shrink fit, adhesive bonding, insert molding, or the like. Since there is a clear boundary between the head drum proper and the surface layer, they are not joined with sufficient bonding strength, or the surface layer is apt to come off easily.